1. Field of the Invention
The present invention relates to an angle sensor system that includes a magnetic field generation unit and an angle sensor.
2. Description of the Related Art
In recent years, angle sensors have been widely used in various applications, such as detection of the rotational position of a steering wheel or a power steering motor in an automobile. The angle sensors generate a detected angle value having a correspondence with an angle to be detected. Examples of the angle sensors include a magnetic angle sensor. An angle sensor system using a magnetic angle sensor is typically provided with a magnetic field generation unit for generating a rotating magnetic field whose direction rotates in response to the rotation or linear movement of an object. The magnetic field generation unit may be a magnet configured to rotate, for example. The angle to be detected by the magnetic angle sensor corresponds to the rotational position of the magnet, for example.
Among known magnetic angle sensors is one that includes a plurality of detection circuits for generating a plurality of detection signals of different phases and generates a detected angle value by performing computations using the plurality of detection signals, as disclosed in JP 2011-158488A. Each of the plurality of detection circuits includes at least one magnetic detection element. The magnetic detection element includes, for example, a spin-valve magnetoresistance element including a magnetization pinned layer whose magnetization direction is pinned, a free layer whose magnetization direction varies according to the direction of the rotating magnetic field, and a nonmagnetic layer located between the magnetization pinned layer and the free layer.
For the magnetic angle sensors, ideally, each of the plurality of detection signals has a waveform of a sinusoidal curve (including a sine waveform and a cosine waveform) when the angle to be detected varies with a predetermined period. However, there are cases where the waveform of each detection signal is distorted from a sinusoidal curve. A distortion of the waveform of each detection signal may result in some error in the detected angle value. The error occurring in the detected angle value will hereinafter be referred to as angular error.
When distorted in waveform, each detection signal contains an ideal component which varies in such a manner as to trace an ideal sinusoidal curve, and an error component other than the ideal component. A detected angle value that is calculated with each detection signal consisting only of the ideal component corresponds to a true angle to be detected by the angle sensor. Such a detected angle value will hereinafter be referred to as ideal angle. Angular error is the difference between the ideal angle and any detected angle value.
The causes of distortion of the waveform of each detection signal are broadly classified into a first cause related to the rotating magnetic field generated by the magnetic field generation unit and a second cause related to the magnetic detection element. In the case of an ideal angle sensor system, when the angle to be detected varies with a predetermined period, a waveform that represents variation in the strength of a component in one direction of the rotating magnetic field at the position of each detection circuit, which will hereinafter be referred to as a field-strength waveform, is sinusoidal, and the waveform of each detection signal generated by each detection circuit is also sinusoidal. The angular error caused by the first cause is due to distortion of the field-strength waveform from a sinusoidal curve.
The angular error caused by the second cause is due to distortion of the waveform of each detection signal from a sinusoidal curve even when the field-strength waveform is sinusoidal. The angular error caused by the second cause is also experienced when, for example, the free layer of a magnetoresistance element, serving as the magnetic detection element has a magnetic anisotropy. Angular error may also be caused by the combination of the first cause and the second cause.
JP 2011-158488A describes a magnetic sensor capable of reducing the angular error caused by the first cause. The magnetic sensor includes a first detection unit and a second detection unit located at positions different from each other. The first detection unit includes a first detection circuit, a second detection circuit, and a first computation circuit for calculating a first detection angle on the basis of output signals from the first and second detection circuits. The second detection unit includes a third detection circuit, a fourth detection circuit, and a second computation circuit for calculating a second detection angle on the basis of output signals from the third and fourth detection circuits. The magnetic sensor further includes a third computation circuit for calculating a detected angle value on the basis of the first and second detection angles.
The magnetic sensor described in JP 2011-158488A requires a large number of detection circuits and computation circuits, which makes the magnetic sensor complicated in configuration.